A fluidized-bed classifier disclosed in JP-A No. Hei 6-343927 adjusts classification particle size (freeboard flow velocity) by adjusting the flow velocity of a gas forming a fluidized bed to separate a mixture of particles into coarse particles held in the fluidized bed and fine particles scattered into the freeboard. The coarse particles are removed from the fluidized bed. A exhaust gas containing the fine particles is extracted from the freeboard and is delivered to a cyclone etc. to collect the fine particles.
It is mentioned in JP-A No. Hei 6-343927 that an auxiliary classifying gas is supplied to a discharge chute for discharging the coarse particles from the fluidized bed to avoid discharging fine particles of particle sizes below the classification particle size together with the coarse particles through the discharge chute. It is also mentioned in this publication that the temperature of the fluidized bed is measured, and the gas forming the fluidized bed is heated so that the measured temperature of the fluidized bed coincides with a temperature necessary for drying the material.
When processing a material, such as coal or slag, by using a fluidized bed, some coarse particles of coal or slag are not fluidized even if a fluidizing gas is blown from below a gas-distributing plate because particle sizes of particles of coal or slag are distributed in a very wide particle size distribution.
According to the fluidized-bed apparatus disclosed in JP-A No. Hei 5-71875, a gas is jetted obliquely upward along the inclined surface of the gas-distributing plate to make coarse particles jump over a jumping board.
A large lump discharging apparatus for discharging large lumps from a fluidized bed disclosed in JP-A No. Hei 6-281110 has a gas-distributing plate disposed in a fluidized-bed furnace and provided with a recess in a central part thereof, and a large lump discharging chute penetrating a wind box and having an upper end connected to the recess.
Generally known gas-distributing plates are of a cap type or of a perforated type.
A cement clinker kiln disclosed in JP-A No. Hei 6-287043 includes a fluidized-bed kiln disposed below the gas distributing plate of a fluidized-bed granulating furnace, and burns cement clinker by supplying grains through a dropping opening facing the fluidized bed of the fluidized-bed granulating furnace into the fluidized-bed kiln. A gas is blown through the dropping opening into the fluidized-bed granulating furnace by a gas blowing means, and fine particles are separated from particles dropped through the dropping opening by adjusting the effective area of the dropping opening by adjusting the position of a classifying gate inserted in the dropping opening through the side wall of the furnace in the dropping opening.
The fluidized-bed classifier disclosed in JP-A No. Hei 6-343927 controls the flow rate of the fluidizing gas to adjust the classification particle size. Since temperature necessary for drying the material changes according to the flow rate of the fluidizing gas (gas flow rate), in some cases, the material cannot be dried in a desired drying degree. In other words, classification particle size and drying degree cannot be simultaneously adjusted because the gas flow rate and the hot air temperature are controlled individually. A satisfactory secondary classifying effect to separate fine particles of particle sizes below the classification particle size cannot be achieved only by supplying the auxiliary classifying gas to the coarse particle discharge chute. Replacement of the abraded or corroded perforated gas-distributing plate with a new one requires much time and great expense. When the material has a wide particle size distribution and contains much large particles, it is possible that the fluidized bed cannot be formed due to the stagnation of large particles in a space directly below a material supply unit.
When the conventional cap type gas-distributing plate is used, a large part of particles remains stationary and large particles do not move and do stagnate. Thus, the cap type gas-distributing plate is unsuitable for handling particles of particle sizes in a wide particle size distribution. Some troubles are caused by abrasion of the cap of the cap type gas-distributing plate and clogging of nozzles. When a perforated gas-distributing plate properly designed taking uniformity in jetting, stationary particles in spaces between nozzles and jetting height into consideration is used, all the material can be fluidized. Such a perforated gas-distributing plate is excellent in resistance to abrasion and clogging. However, a relatively large amount of the material drops through the perforated plate and deposits in the wind box.
The fluidized-bed apparatus disclosed in JP-A No. Hei 5-71875 needs to jet the gas at a very high velocity. Therefore, pressure loss in the fluidized-bed apparatus is great, the gas-distributing plate is abraded rapidly and the replacement of the gas-distributing plate with a new one takes much time and needs great expense. The gas-distributing plate of complicated construction requires complicated, troublesome maintenance work. Since a maximum particle size, i.e., the particle size of particles that can be carried, is dependent on gas jetting velocity, it is possible that large particles stagnate on the gas-distributing plate and stop the operation of the fluidize-bed apparatus. The velocity of the fluidized bed must be increased to ensure the conveyance of coarse particles and, consequently, the amount of scattered fine particles increases.
The large lump discharging apparatus for discharging large lumps from a fluidized bed disclosed in JP-A No. Hei 6-281110 discharges large lumps through the respective central parts of the gas-distributing plate and the wind box is complicated in construction and is incapable of surely discharging large lumps. Therefore, the deposition of large lumps increases with time and, eventually, the fluidity of the fluidized bed is deteriorated.
The cement clinker kiln disclosed in JP-A No. Hei 6-287043 employing the classifying gate disposed in the bottom of the fluidized-bed granulating furnace make particles float in a gas flow from the bottom of the granulating furnace. Since classifying gas velocity for separating fine particles is low, particles flow altogether into the classifying part of the chute and fill up the classifying part. Consequently, the apparatus is unable to fully exercise its classifying effect.
The present invention has been made in view of the foregoing problems. Therefore, it is an object of the present invention to provide a fluidized-bed drying and classifying apparatus which is capable of maintaining a satisfactory, stable fluidized bed when drying and classifying particles of a material having a wide particle size distribution, such as coal or slag, by using the fluidized bed and of adjusting both drying degree and classification particle size, and is simple in construction, inexpensive, safe, and easy to operate and maintain.
Another object of the present invention is to provide a fluidized-bed drying and classifying apparatus which is capable of operating at an improved classifying efficiency by greatly reducing the fine particle content of coarse particles, i.e., processed material, of maintaining a stable fluidized bed even if the material contains a large amount of coarse particles and lumps and of surely preventing the inclusion of large lumps in a processed material.